Limiting amplifier



May 19, 1942.

C. C. VAN COTT LIMITING AMPLIFIER Filed Feb. 14, 1940 2 Sheerls-Sheet 1r 0 P w 0 e F 7. 7 mm a; F Fl n M 5 r 7 M Av v \r 4 4 mp PW W 3 i WM. 3.2 Z 6% W MI L w I; W o

.LEr- E 22% .W 2 WWW p w a 0 a m? 6 May 19, 1942.

rs. c; VAN COTT LIMITING AMPLIFIER 2 Sheets-Sheet 2 7 Filed Feb. 14,1940 POWER +4 +6 +8 +10 5%0 (140 (9 wa CPO U;

' 14 ZZorzzey Patented May 19, 1942 LIMITING AMPLIFER Charles C. VanCott, Soleta, Calif., assignor to Collins Radio Company, 'Gedarcorporation of Iowa Rapids, Iowa, a

Application February 14, 1940, Serial No. 318,957

6 Claims.

My invention relates broadly to' gain control systems for electron tubeamplifiers, and more particularly to a limiting amplifier employingvarying impedance electron tubes and inverse feedback for automaticoperation.

One of the objects of my invention is to provide an improved limitingamplifier of low distortion characteristics and substantially uniformresponse over the audio frequency range with standard componentelements.

Another object of my invention is to provide a circuit arrangement forinverse feedback wherein the impedance of an electron tube or tubes isvaried by control of grid potential in limiting the output of an audiofrequency amplifier under input voltage peaks. A further object of myinvention is to provide a circuit arrangement for remote or automaticgain control in a limiting amplifier through inverse feedback.

Still another object of my inventionis to provide a limiting amplifierwherein inverse feedback is-efiected'through the impedance of anelectron tube' circuit arranged for compressing the output voltage waveon the occurrence of voltage peaks in the input.

A still further object of my invention is to provide a circuitarrangement for a limiting amplifier employing inverse feedback from theoutput of the amplifier through the'impedance of an auxiliary electrontube circuit.

Other and further objects of my invention reside in the circuitarrangements hereinafter described, with reference to' the accompanyingdrawings, in which:

Figure 1 is a block diagram indicating the general arrangement of theportions of the system of my invention; Fig. 2 is a schematic diagram ofone embodiment of my invention; Fig. 3 is a schematic diagram of amodified form of my invention for a push-pull amplifier circuit; andFig. 4 is a graph showing performance characteristics of the system ofmy invention shown in Fig. 3.

The system of my invention affords a simplified arrangement forthecontrol of inverse feedback in an electron tube amplifier for limitingthe output voltage upon the occurrence of voltage peaks in the input.Within the normal range of operation, for input voltages below apredetermined level, the amplifier functions substantially withoutcompression, but the limiting elements of the circuit of my inventionfunction efliciently exceeded, to control the output voltagewith aminimum of distortion and circuit readjustments sothat an output wave ofdesired 'fidelity and regularity is delivered. Losses are minimized, andthe assembly may be constructed compactly and light in weight forvarious useful adaptations, such as in aircraft and portable units.

In Fig. 1, the limiting amplifier is "shown to coupling stage at 3operates under the influence of a control at 4,'supplied with a portionof the output energy, to supply feedback energy to the controlled stageI inversely as the output voltand efiectively when the predeterminedlevel is age'varies above a predetermined level. Suitable delayprovisions are made in a portion of .the control circuit for properregulation.

Referring specifically to the form of my invention shown in Fig. 2, thecontrolled amplifier at I is shown with a potentiometer 5 in the inputcircuit and resistance coupling of the output circult to the bufferamplifier 2 in conventional manner, the output of amplifier 2 beingdelivered through transformer B. A portion of the output voltage ofamplifier 2 i applied through condenser I and potentiometer 8 to thegrid of a selfbiased reactifier 4, and the resulting unidirectionalcontrol current in the anode circuit of the tube 4 flows throughresistor 9' connected with the cathode thereof. Coupling tube 3' has agrid circuit which includes a filter I0 connected with the resistor 9, asource of bias potential H and a resistor I2 which is a load resistor inthe anode circuit of the. tube 3. The bias source I I is in circuit alsowith the cathode and grid of rectifier 4 and thus affects bothelectrodes equally; the source II is of the same relative polarity asthe .anode source of'power I4 in respect to the rectifier 4.

Coupling tube 3 is constituted as a variable high impedance deviceconnected between the output circuit of tube 2 and resistor I2. The biaspotential for tube 3, from source I I, is such that current isblocked-up tothe point at which limiting is to begin, at which point thepotential drop across resistor 9 is sufficient to apply a net potentialto the grid of tube 3, through-filter III," to produce current in tube 3and in resistor I2 which 'results in a. voltage thereacross. Resistor I2is connected with the control grid of amplifier I, in

shunt with the cathode bias resistor I5 through to the-control grid tolimit the output voltage.

The compression resulting from this operation is adequate toproduce thedesired output voltage,

but the non-linearity, thus introduced does not materially distort theoutput wave. The condenser I6 and resistor I2 may be omitted if desiredand the cathode connection from tube 3 made directly to the cathodeterminus of resistor I 5, with the same results. The filter I 0 isdesigned with the'desired delay characteristic for preventing unduefluctuation in the control effect. As above noted, the point at whichlimiting will begin may be regulated by'the bias potential on the gridof tube 3, obtained from the source II. The amount of compressionafforded is determined by the position of the tap connection to 2 thegrid of tube 4 on the potentiometer 8. And the input potentiometer 5controls the signal voltage applied to the limiting amplifier Theresistances and capacitances in the remainder of the circuit areconventional elements and of such character that an assembly embodyingthe limiting amplifier of my invention may be constructed compactly andlight in weight. An audio frequency choke I1 is provided in the anodepower supply connection to the buffer amplifier 2 and coupling tube 3,beyond the connection of rectifier 4 to the power supply |4.

The modified form of my invention illustrated in Fig. 3 employs a doublerectification stage in order that a filter having a smaller timeconstant may be employed for more alert limiting action in the limitingamplifier. Double rectification results in a large proportion of directcurrent at the cathode of the control rectifier with only higherfrequency distortion components to be filtered out, which components maybe blocked by a smaller resistance-capacitance filter, causing theamplification to drop very fast on sudden large signals. 7

In Fig. 3, a limiter cut-out switch I8 permits the input line to bedirectly connected with the output, but in the position shown switch l8connects the input through a resistance pad l9 to an input transformer20. A push-pull limiting amplifier stage 2|, 2|, corresponding to thelimiting amplifier I, is energized from the input transformer 20 andresistance coupled with a push-pull buffer amplifier 22, 22', whichcorresponds to buffer amplifier 2, in conventional manner. The inversecoupling stage comprises electron tubes 23 and 23 associatedrespectively with the opposite portions of the push-pull limitingamplifier 2|-22, 2|-22, similarly as the coupling stage 3 of Fig. 2 inrespect to limiting amplifier |-2. The rectifier and filter 4 of Fig. 2is represented by a control amplifier tube 24 which, by virtue of a highnegative grid bias from a source at 25, functions as a half-wave squarelaw amplifier, or primary rectifier; in cooperation with tube 24 is afull wave secondary rectifier 26, the output of which is suppliedthrough a filter, consisting of resistor 21 and condenser 28, to thegrids of tubes 23, 23 in parallel.

The output of buffer amplifier 22, 22" is delivered through outputtransformer 29, but a portion of the output from one of the tubes 22',is applied through coupling condenser to the grid of the controlamplifier tube 2.4. Potenfor regulating the amount of compressioneffected in the limiting amplifier 2|, 2|. The rectified output of thesquare law half-wave amplifier 24 is applied through transformer 32,without filtration, to the full .wave rectifier 26 and the resultingdirect current in a load resistor 33 produces a voltage therein which isapplied to the grids of tubes 23, 23, through the filter 27, 28. Thepotential on the grids of tubes 23, 23' comprises the control potentialfrom resistor 33, and a bias potential derived from the source at 25through a potentiometer 34 which is adjusted to a value at which a lossin gain is first noted, with zero potential difierence on the resistor33.

The input to each of the tubes 2|, 2|, is controlled by a separatepotentiometer 35, 35', respectively, adjusted for a degree ofamplification to overcome the loss in the resistance pad l9 or,alternatively, to control the amplification when the pad I9 is notemployed. Audio frequency tiometer 3| is provided in the couplingconnection choke 36 is provided in the anode power supply connectionfrom source 3'! to control amplifier tube 24.

Similarly as in the arrangement of Fig. 2, the tubes in the couplingstage of the limiting amplifier are constituted as high impedance valvesin circuit with the outputs of the respective buffer stages andresistors-connected in the cathode circuits of the limiting stages. InFig. 3, tube 23 is in circuit with resistor 38 and conducts feedbackcurrent from the output'of tube 22 -in accordance with the potential onthe grid thereof resulting from operation of the control amplifier 24and rectifier 26. Tube 23' is similarly related to limiting stage 2|through the cathode resistor 38'. E

In brief, then, the operation of the limiting amplifier of my inventionis as follows, referring to Fig. 3. Tubes 23, 23', are constituted asvariable resistance elements for passing current from theoutput'circuits of tubes 22, 22', through the input circuit resistances38, 38', a limiting action inthe amplifiers 2 I, 2| being effected byinverse feedback with increase of feedback current in the impedancetubes 23, 23. The grids of tubes 23, 23' are normally maintained at highnegative potential, adjusted by potentiometer 34, but the potential isvaried by a rectified potential in' positive polarity, at resistor 33,produced by rectification of alternating current derived in transformer32 from half wave pulsations delivered from control amplifier 24. Thetube 24 controls the amount of gain reduction that takes place at agiven voltage levelin accordance with the magnitude of .the controlpotentials derived from the amplifier 22 and adjusted in potentiometer3|. Tube 24 delivers uni-directional current impulses at the selectedsignal level. Such current impulses produce alternating impulses in thesecondary of transformer 32 which are then rectified and emerge withsubstantially little distortion as a direct current in the resistor 33.A positive potential from the resistor 33 applied to the grids of thetubes 23, 23', decreases the in pedance thereof, thus increasing thefeedback currents in resistors 38, 38'.

Fig. 4 shows three curves representing operating conditions achievedwith the arrangement shown in Fig. 3. Curve A with little compressionshows rapid limitation of the output voltage above a certain level; sucha curve is useful in the operation of a broadcast transmitter as asafeguard against overmodulation when compression is not desired atnormal operating level. Curve B allows a moderate degree of compressionat normal level with limitation on peaks to prevent overmodulation."Curve C may be used for automatic level control for a voltage source,such as a speech channel to hold the output constant for variousspeakers voices and at differing microphone distances. Curve A isobtained with highest threshold bias voltage on the a less negativepotential on the grids of tubes 23 and 23'; for curve B, the controlsare respectively and 60% positions; and for curve C, both the controls3| and 34 are at 50%positi0 ns.

With the circuit shown in Fig. 3, frequency response uniform with onedecibel from 30 to 12,- 000 cycles is obtained with inexpensive standardcircuit components due to the improvement in frequency response achievedby the limiting action employed in my invention. This limiting action iseffected by inverse feedback through electron tubes of varyingimpedance, as explained. The internal resistance of these tubes is a'function of the bias potentials on their grid electrodes, and thevariation of feedback current in the tubes corresponds to the change inresistance. The control voltage, having been derivedafter'rectificationof the signal voltage and filtered to remove harmonic anddisto rtioncomponents, is essentially a direct current voltage and thus does notaffect the signal componentsin the limiting amplifier, v I

Double rectification is a valuable feature becircuit with said secondaryrectifier, and a circuit connected with both said control electrodesincludingjsaid resistor and a source of bias pocause the higherfrequency of the distorted sig nal components and large proportion ofdirect current at the cathode of control rectifier 26 al- I cursion ofthe grid of control tube 24 to coincide tube, and means for biasing saidgrid electrode, 1 t

reaches 1 with the negative modulation excursion of the transmitter,negative peak protection is afforded in the case of asymmetric signalwaves with the greatest peak extending in a negative direction.

If the direction of asymmetry extendsin a positive direction thehalf-wave feature allows the level reduction to be determined by theminor side of the wave, giving a level increase equal to the ratioofpeak asymmetric of the wave.

It should be noted that the cathode resistance I5 of tube l in Fig. 2,and cathode resistances 38, 38', of tubes 21 and 2| in Fig. 3, cannot beby-passed to ground through a large condenser, such as is shown in thecathode circuit of tube 2. The signal voltage fed back from the anode oftube 2, for example, through the anode-cathode resistance path of tube 3must appear across the cathode resistance of tube I. thus introducedinto the cathode circuit of tube l and appearing across resistance l5is'effectively impressed across the grid and cathode of tube I and isaccordingly reintroduced into the input of the signal channel in themanner necessary for inverse' feedback, whereas a by-pass condenserwould conduct the-audio currents around the resistance l5, Fig. 2.

While I have described my invention in certain preferred embodiments, Idesire it understood Any audio voltage tential connected in series, withthe potential drop in said resistor in opposite polari With respect tothe voltage of said source, for varying the impedance of each saidelectron discharge device in accordance with the amplitude of the signalvoltage above a predetermined level, for

simultaneously regulating the feedback energy supplied to both portionsof said push-pull amplifier through said impedances.

2. A limiting amplifier asset forth in claim 1 a with sai'd pprimaryrectifier comprising a triode electron tube, a potentiometer in serieswith the output of said buffer amplifier with a portion of the voltagethereacross impressed between cathode and rid electrodes in said triodeelectron the potential on said grid electrode determining the magnitudeof the resulting feedback in the limiting amplifier at a given signalvoltage.

3. A limiting amplifier as set forth in claim 1 with filter sansinterposed in the circuit connected with both said control electrodes,said filter means eing adapted to suppress high frequency distortion andharmonic components inthe output of said secondary rectifier andinterposing substantially no delay in the application of controlcomponents of potential tosaid control electrodes.

4. A limiting amplifier as .set forth in claim 1 with apotentiometenconnected with said source of bias potential for adjustingthe magnitude 0 the bias potential applied to said control electrodes,for selectively determining the level at which limiting begins.

5. In a limiting amplifier, .an electron tube.

amplifier including a cathode resistor, a variable impedance connectedin circuit'yvith said cathode resistor and the output of said amplifierfor inverse feedback operation, a primary rectifier energized from theoutput of said electron tube amplifier, a secondary rectifier, atransformer for coupling said primary and secondary rectisaid primaryrectifier, a resistor in circuit with 1 said'secondary rectifier forproducing a voltage that further modifications may be made within thescope of my invention, and that no limitations upon my invention areintended except as are imposed by the appended claims.

1. A limiting amplifier comprising a push-pull electron tube amplifierhaving a separate cathode resistor in each portion thereof, a bufferpushpull amplifier coupled with the output of said What I claim as new.and desire to secure by Letters ,Patent of the United States is asfollows;

electron tube amplifier, a separate inversei'eedtransformer for couplingsaid primary and sec:

ondary rectifiers and energized by current pulse- I tions from saidprimary rectifier, a resistor in proportional to the amplitude of thesignal volt,-* age 7 above a predetermined level, and means f.energized'by said voltage for varying said im-' pedance for regulatingthe feedback energy supx plied to said electron tube amplifier throughsaid impedance. i 6. In a limiting amplifier, an electron tube amplifierincluding a cathode resistor, an electron discharge device connected asa variable impedance in circuit with said cathode resistor and theoutput of said amplifier for inverse feed-- back operation, saidvariable impedance electron discharge device having a control electrode,a

primary rectifier energized from the output of I said electron tubeamplifier, a secondary rectifier, a transformer for coupling saidprimary and secondary rectifiers and energized by current pulsationsfrom said primary rectifier, a resistor in circuitwith said secondaryrectifier for producing a potential proportional to the amplitude ofthesignal voltage above a'predetermined level, and means for applying saidpotential to said control electrode forvarying the impedance of saidelectron discharge device and regulating the feedback enereysupplied tosaid electron tube.

amplifier through said impedance.

CHARLES C. VAN'COTT.

